In a crossed aldol condensation (condensation between aldehyde and a ketone), the primary products are such, that the carbonyl group of the ketone remains intact, and the $\alpha$-hydrogen of ketone participates, i.e. gets removed by the base to form the resonance stabilised nucleophilic enolate which later attacks the aldehyde. These are formed together with the self-condensation products, but the reverse products (aldehyde-derived enolate + ketone) are rarely formed.

I had a look at the mechanism, and also how a base catalyses condensation by favouring enolate formation.

Why is the ketone involved in enolate formation and not aldehyde in crossed/mixed aldol-condensation reactions?

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    $\begingroup$ Your example might not be very well chosen since in the given case, the aldehyde doesn't contain any $\alpha$-hydrogens, so it can't be enolized. $\endgroup$ – Philipp Nov 28 '13 at 19:05
  • $\begingroup$ @Philipp I did not have my scanner yesterday. The only example I could post was what I found on the net, which was unfortunately this. $\endgroup$ – stochastic13 Nov 29 '13 at 1:08

Ketones have two alkyl groups attached to the carbonyl moiety, which are electron-donating by hyperconjugation. That is, the overlap of $\ce{C-C}$ and $\ce{C-H}$ $\sigma$-bonds with the $\pi$-type molecular orbitals of the carbonyl increases the electron density around the carbon of the carbonyl group, thus making it less electrophilic.

Alternatively, I think it's also equally (if not more) valid to consider the reactivity from the perspective of the frontier orbitals. Alkyl substituents tend to slightly lower the energy of the HOMO of the carbonyl (by comparison to hydrogens), but concomitantly they raise the energy of the $\pi^{*}$ LUMO of the carbonyl, into which the electrons of the enolate would flow, resulting in lower electrophilicity (since the energy of the tetrahedral intermediate and transition state resulting from attack by the enolate are going to be correspondingly higher).

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